Supramolecular Pseudorotaxane Polymers from Complementary Pairs of Homoditopic Molecules

Abstract

Self-assembly of supramolecular pseudorotaxane polymers from complementary homoditopic building blocks comprised of bis(dibenzo-24-crown-8) esters derived from the hydroxymethyl crown ether and aliphatic diacid chlorides (CxC, x = number of methylene units in the diacid segment) and 1,10-bis[p-(benzylammoniomethyl)phenoxy]alkane bis(hexafluorophosphate)s (AyA, y = number of methylene units in the linker) has been studied. (1)H NMR spectroscopic studies of bis[(2-dibenzo-24-crown-8)methyl] sebacate (C8C) with dibenzylammonium hexafluorophosphate (6) showed that the two binding sites of the ditopic host are equivalent and independent (no positive or negative cooperativity). Likewise the binding sites in 1,10-bis[p-(benzylammoniomethyl)phenoxy]decane bis(hexafluorophosphate) (A10A) were shown to behave independently with dibenzo-24-crown-8 (1a). Then using (1)H NMR spectroscopy on dilute equimolar solutions (<1 mM) of CxC and AyA association constants were estimated for the formation of the linear (lin-CxC*AyA) and cyclic (cyc-CxC*AyA) dimers, thus enabling effective molarities to be estimated for the various systems. Finally (1)H NMR spectroscopy was used to semiquantitatively or qualitatively demonstrate the formation of linear supramolecular polymers lin-[CxC*AyA](n) in more concentrated solutions (up to 2.0 M) of the complementary pairs of CxC and AyA. The sizes of the assemblies (n values) are not as great as the dilute solution studies predict; this is attributed to the deleterious effect of ionic strength and exo complexation at high concentrations. However, as expected from the dilute solution results, linear extension is indeed favored with the longer building blocks, meaning that "monomer" end-to-end distance is a key factor in reducing the amount of cyclic species that form. Viscosity experiments clearly demonstrate the formation of large noncovalent polymers lin-[CxC*AyA](n) in concentrated solutions. Cohesive film and fiber formation also indicate that supramolecular polymers of sufficient size to enable entanglement self-assemble in these solutions.